The present invention relates to light emitting diodes, and in particular relates to light emitting diodes (LEDs) that are used in conjunction with a phosphor that converts light from the LED to produce an output that is either partially or totally a combination of the frequencies emitted by the LED and those converted by the phosphor.
Light emitting diodes are a class of photonic devices in which the application of current across the device, and most fundamentally across p-n junction, generates recombination events between electrons and holes. In turn the events produce at least some energy in the form of emitted photons.
Because the recombination events are defined and constrained by the principles of quantum mechanics, the energy (and thus the photon) generated by the event depends upon the characteristics of the semiconductor material in which the event takes place. In this regard, the bandgap of the semiconductor material is the most fundamental characteristic with respect to the performance of light emitting diodes. Because the recombination events take place between the valence band and the conduction band of the semiconductor materials, they can never generate an amount of energy larger than the bandgap. Accordingly, materials with smaller bandgaps produce lower energy (and thus lower frequency) photons while materials with larger bandgaps can produce higher energy, higher frequency photons.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, generally low cost.
Light emitting diodes, or at least the light emitting properties of semiconductors, have been recognized for decades. A 1907 publication (H. J. Round, Electrical World 49, 309) reported that current applied through silicon carbide produced an observed but unexplained emission of light. More widespread commercial use of LEDs began in the 1970s with indicator type use that incorporated lower frequency LEDs (typically red or yellow in color) formed from smaller bandgap materials such as gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP).
In the 1990s, the development of the blue light emitting diode as a commercial rather than theoretical reality greatly increased the interest in LEDs for illumination purposes. In this regards, “indication” refers to a light source that is viewed directly as a self-luminous object (e.g. an indicator light on a piece of electronic equipment) while “illumination” refers to a source used to view other objects in the light reflected by those objects (e.g., room lighting or desk lamps). See, National Lighting Product Information Program, http://www.lrc.rpi.edu/programs/NLPIP/glossary.asp (December 2006).
Although light emitting diodes have become widely adapted for indicator purposes, their potential use for illumination includes applications such as indoor and outdoor lighting, backlighting (e.g. for displays), portable lighting (e.g., flashlights), industrial lighting, signaling, architectural and landscaping applications, and entertainment and advertising installations.
The availability of blue light emitting diodes correspondingly provides the opportunity for at least two techniques for producing white light. In one technique, blue LEDs are used in conjunction with red and green LEDs so the combination can form white light or—such as in a full-color display—any other combination of the three primary colors.
In a second technique, and one that has become commercially widely adopted, a blue light emitting diode is combined in a lamp with a yellow-emitting phosphor; i.e., a phosphor that absorbs the blue light emitted by the LED and converts and emits it as yellow light. The combination of blue and yellow light will produce a tone of white light that is useful for many illumination circumstances.
Although the terminology is used flexibly, the word “diode” (or “light emitting diode”) is most properly applied to the basic semiconductor structure that includes the p-n junction. The term “lamp” most properly refers to a packaged device in which the diode is mounted on electrodes that can connect it to a circuit and within a polymer lens that both protects the diode from environmental exposure and helps increase and direct the light output. Nevertheless, the term “LED” is often used to refer to packaged diodes that might more correctly be referred to as lamps and vice versa. Generally speaking the meaning of the terms will be clear in context.
Because the blue frequencies represent the highest energy within the visible spectrum (with red frequencies being the lowest), they must be produced by relatively high-energy recombination events. This in turn requires that the semiconductor material have a relatively wide bandgap. Accordingly, candidate materials for blue light emitting diodes, and thus for white-emitting LED lamps, include silicon carbide (SiC), the Group III nitrides (e.g., GaN), and diamond.
Although the understanding and production of silicon carbide has increased greatly in the last 20 years, in quantum terms it is an indirect emitter. This means that when a recombination event takes place in SiC, a significant amount of the energy is emitted as a vibration rather than as a photon. Thus, although silicon carbide produces blue photons, it does so less efficiently than it would if it were a direct emitter.
Accordingly, most interest in blue light emitting diodes has focused upon the Group III nitride materials such as gallium nitride, aluminum gallium nitride (AlGaN), and indium gallium nitride (InGaN). The Group III nitrides have bandgaps sufficient to produce blue photons and are direct emitters, thus increasing their efficiency.
Illumination, however, tends to require higher quantities of light output than does indication. In this regard, the number of individual photons produced by a diode in any given amount of time depends upon the number of recombination events being generated in the diode, with the number of photons generally being less than the number of recombination events (i.e., not every event produces a photon). In turn, the number of recombination events depends upon the amount of current applied across the diode. Once again the number of recombination events will typically be less than the number of electrons injected across the junction. Thus, these electronic properties can reduce the external output of the diode.
Additionally, when photons are produced, they must also actually leave the diode and the lamp to be perceived by an observer. Although the majority of photons will leave the lamp without difficulty, a number of well-understood factors come into play that prevent the photons from leaving and that can thus reduce the external output of an LED lamp. These include internal reflection of a photon until it is re-absorbed rather than emitted. The index of refraction between the materials in the diode can also change the direction of an emitted photon and cause it to strike an object that absorbs it. The same results can occur for yellow photons that are emitted by the phosphor. In an LED lamp such “objects” can include the substrate, parts of the packaging, the metal contact layers, and any other material or element that prevents the photon from escaping the lamp.
To date, bulk crystal growth of large Group III nitride crystals remains difficult. Accordingly, in order to form the thin, high quality epitaxial layers that produce p-n junctions in LEDs, the Group III nitride materials must typically be grown on a substrate. When, as in some constructions, the substrate remains as part of the eventual light emitting lamp, it can provide one more opportunity to absorb photons emitted by the junction, thus reducing the external quantum efficiency of the overall diode.
In short summary, a number of factors can reduce the external light output of an LED lamp. Accordingly, a need exists for continued improvement in increasing the external output of such LED lamps.